The field of the present invention relates generally to diamond coatings for cutting tools and wear parts, and more particularly to a polycrystalline diamond coating including a graded diamond layer having a progressively finer grain size in the direction of the outer surface for providing enhanced wear resistance and smoother finishing characteristics.
There is an increasing demand for harder, more abrasion resistant cutting tools. Recent advances in material science have led to the development and widespread use of extremely hard and abrasive materials such as improved ceramic materials, metal matrix composites, silicated aluminum, graphite composites, fiber reinforced plastics or the like. This has created a heightened demand for abrasion resistant cutting tools which are capable of machining the new materials.
Conventional cemented carbide cutting tools, which are typically coated with a material such as titanium nitride (TiN) or titanium carbide (TiC) or a combination of the two for enhancing performance, are no longer adequate for machining modern abrasive materials. It has been found that diamond cutting tools last at least ten times longer than conventional coated carbide tools. However, conventional diamond tools also cost at least ten times as much as carbide tools. Thus, tool cost is presently a disadvantage of conventional diamond cutting tools.
The hardness and thermal properties of diamond are but two of several characteristics that make diamond useful in a variety of industrial applications. Diamond may be synthesized by high pressure-high temperature (HP-HT) techniques utilizing a catalyst/sintering aid where diamond is the stable phase. This process has been used to form polycrystalline diamond (PCD) compacts which can be bonded or fastened to a supporting body, often of tungsten carbide, to form polycrystalline diamond tools.
A variety of work has been done in this field focusing upon the use of binders and the coating of diamond particles to retain diamond grit and to improve wear resistance. See, e.g., U.S. Pat. Nos. 5,024,680 and 5,011,514, and references discussed therein as examples of conventional methods for improving grit retention in a matrix by metal coating diamond particles. In other conventional methods, layers of binder material are used between diamond and the supporting tool or substrate to improve bonding and adhesion. See U.S. Pat. No. 4,766,040 ("Hillert") and references discussed therein.
One of the problems in a conventional method of forming a diamond coating over a tool is that adhesion may be hindered due to a thermal expansion mismatch between the supporting tool and the hard, rigid polycrystalline diamond working edge. To overcome this problem, Hillert uses multiple layers of diamond with different levels of a low-melting point binding metal. The composition of the layers is varied such that the thermal expansion of the layers is higher for internal layers near the supporting tool, while the outer working edge is harder and more rigid. Hillert describes that preferably the metal concentration of the polycrystalline diamond body is decreased towards the working surface. Thus, multiple interlayers are used to improve the bonding between a supporting tool and a hard, rigid diamond working edge. The Hillert patent does not teach the use of a fine grained coating to alter the properties of the working edge. The properties of the working edge may be altered to some extent, however, by altering the type and amount of binder used as well as the size of the diamond particles. For instance, U.S. Pat. No. 4,171,973 describes the use of very fine diamond particles with a binder to improve the surface finish of a sintered diamond compact. However, the diamond grains are essentially glued using high levels of a cobalt binder. This has the disadvantage of reducing wear resistance and hardness.
Another disadvantage of polycrystalline diamond tools is that such tools are costly to manufacture. Also, due to high pressure and high temperature fabrication requirements, polycrystalline diamond material must be manufactured as a flat slab of material having a thickness typically 1 mm or more. Thus, polycrystalline diamond slabs are not adaptable to tools having complex shapes such as chip groove inserts, taps and drill bits.
To overcome the foregoing disadvantages and problems of conventional methods of providing a diamond cutting tool, efforts in the industry have focused upon the growth of adherent diamond films at low pressure, where it is metastable. Although low-pressure techniques have been known for decades, improvements in growth rates have made the process a commercially viable alternative to polycrystalline diamond compacts.
Low pressure growth of diamond is accomplished through chemical vapor deposition (CVD). Three types of CVD are typically used for diamond growth, hot filament CVD, plasma torch, and plasma-enhanced CVD (PECVD). A variety of work has been done with all three techniques to improve growth rates, uniformity of the diamond film, reduction of defects and non diamond impurities, and epitaxial growth on diamond or non diamond substrates (S. Lee, D. Minsek, D. Vestyck, and P. Chen, Growth of Diamond from Atomic Hydrogen and a Supersonic Free Jet of Methyl Radicals, Science, Vol. 263 at 1596 (Mar. 18, 1994)). The following patents address many of the problems inherent in low pressure growth of diamond: U.S. Pat. No. 5,112,649 (improved filament for longer process duration in hot filament CVD), U.S. Pat. No. 5,270,077 (method of producing flat CVD diamond film primarily for use in electronics), U.S. Pat. No. 5,147,687 (hot filament CVD of multiple diamond layers to provide thick coatings), and U.S. Pat. No. 5,256,206 (CVD of uniform film on irregular shaped objects such as twist drills).
Adequate adhesion of a diamond layer to a substrate or tool also has been an obstacle to the use of diamond films. U.S. Pat. No. 4,842,937 describes a conventional method for providing a polycrystalline diamond coating similar to the method described in Hillert. A plurality of layers are deposited on a cutting tool using CVD or other techniques known in the art. Each successive layer disposed further from the base has a higher modulus of elasticity and a greater diamond constituency than the preceding layer. The outermost layer is polycrystalline diamond. As with Hillert, this layering is used to enable a hard, rigid diamond layer to be used as the working edge.
U.S. Pat. No. 5,236,740, which is hereby incorporated by reference, specifically addresses the problem of coating cemented tungsten carbide substrates with adherent diamond films. Cemented tungsten carbide can be formed into a variety of geometries and has the requisite toughness to be a very desirable substrate for the deposition of adherent diamond films.
Despite these advances in the field of diamond tooling, there are still many problems that have not been adequately addressed. First, conventional CVD diamond tools have a rough surface which is not desirable for fine cutting and machining because of the resulting poor surface finish of the machined workpiece. Polishing of the diamond working edge and similar techniques may be used to smooth the surface of the cutting tool, but this is costly and labor intensive. While grain size may be reduced in polycrystalline diamond compacts, or the growth of diamond may be controlled in CVD processes to some extent, it is desirable to find an inexpensive and effective method to reduce the surface roughness of diamond tools, particularly cemented tungsten carbide tools coated with an adherent diamond film.
Also, what is needed is a method to improve the wear resistance of diamond coated tools. A conventional large grain diamond coating has a naturally rough edge which provides many opportunities for crack formation and propagation which can cause premature tool failure. Preferably, such a method also would reduce the formation and propagation of cracks in the diamond.
What is also needed is a smoother diamond coating to reduce the adhesion of workpiece material to the tool surface during the machining process. A smoother tool advantageously results in a lower amount of friction between the workpiece and the tool. This reduces the transfer of heat and improves the wear rate of the tool.
It is extremely labor intensive to polish a conventional diamond tipped or coated tool, and this would add disproportionately to the cost of such a tool. Also, in a situation wherein the geometry of the tool is complex, it is not practical to polish a diamond coated tool in order to make the tool surface smooth.